SELF-REGULATING FILM HEATER

Abstract

 A film comprising,
a first layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/10min or more (measured according to ISO1133 at 125°C under a load of 21.6 kg) and a conductive filler; and
a second layer comprising a polyolefin with a melting point at least 25°C higher than the melting point of the polyethylene of the electrically semiconductive composition, such as a polypropylene homopolymer, a polypropylene copolymer, or a high density polyethylene, preferably a heterophasic polypropylene copolymer; and
a plurality of conductors in contact with or embedded within the first layer.

Description

[0001] This invention relates to a self-regulating film heater and to processes for the preparation of such a structure. In particular, the invention relates to a multilayer film comprising a plurality of conductors. The multilayer film comprises a first layer comprising a semiconductive composition and a second layer comprising a higher melting point polyolefin, such as a heterophasic polypropylene copolymer or high density polyethylene. The film is also provided with a plurality of conductors and can, inter alia, be prepared by coextrusion, e.g. by cast or blown film extrusion processes.

Background of Invention

[0002] Parallel resistance self-regulating heating cables are known. Such cables normally comprise two conductors extending longitudinally along the cable. Typically, the conductors are embedded within a resistive polymeric heating element, the element being extruded continuously along the length of the conductors. The cable thus has a parallel resistance form, with power being applied via the two conductors to the heating element connected in parallel across the two conductors. The heating element usually has a positive temperature coefficient of resistance. Thus as the temperature of the heating element increases, the resistance of the material electrically connected between the conductors increases, thereby reducing power output. Such heating cables, in which the power output varies according to temperature, are said to be self-regulating or self-limiting.

[0003] Thus, to avoid overheating and potential destruction of the object, they are self-limiting and require no regulating electronics.

[0004] Self-regulation utilises a conversion from electrical to thermal energy by allowing a current to pass through a semiconductive medium with Positive temperature coefficient (PTC) characteristics, which elevates the object temperature above that of its surroundings, until a steady state is reached (self-regulation). A material with a PTC has an electrical resistance that increases with temperature, and is the mechanism behind the self-regulating function. These PTC cables are often used in underfloor heating or wrapped around pipes for e.g. anti-freeze purposes. Cables however, do not offer a significant surface area of heat so it takes a large number of cables to provide underfloor heating for example.

[0005] The use of flat sheet self-regulating heaters is also known therefore. In WO2014/188190, an electrical heater is described that comprises conductors and a heating element disposed between the conductors wherein the heating element comprises an electrically conductive material distributed within a first electrically insulating material. The insulating material separates the conductor from the electrically conductive material.

[0006] US7250586 describes a surface heating system for a car seat or the like comprising a support and a heating layer that contains an electrically conductive plastic, which is characterized by the fact that the heating layer is formed by a flexible film and that the support is flexible.

[0007] US4247756 describes a heated floor mat in which two inner electrically conductive inner layers sandwich conductors. These conductors are adhered to the inner layers.

[0008] US7053344 describes a flexible heater for a fabric.

[0009] US2021112631 describes a heating element that has at least one film of an electrically conductive polymer material.

[0010] US5451747 discloses a heat mat with PTC material.

[0011] WO2008/133562 describes a heating device comprising two elongated electrodes arranged at a distance and being inter-connected by a semiconducting heat generating member of a polymer based material having positive temperature coefficient regarding resistivity (PTC-material), wherein the heat generating member comprises electrode interconnection sections of a low resistivity PTC material compared with the PTC material of intermediate section.

[0012] EP1275274 describes a device for floor heating comprising a bendable, electrically conductive, thermoplastic mat. The device is provided with at least two electrodes. Current is conducted through the device, which heats up and emits heat.

[0013] WO2021/188595 describes a blanket comprising a first outer panel, a self-regulating heating element proximate to said first outer panel and a second outer panel proximate to said self-regulating heating element joined to said first outer panel wherein said first outer panel and said second outer panel contain said self-regulating heating elements.

[0014] WO2022/129251 describes a self-regulating flat sheet heater prepared by coextrusion where conductors are embedded within the semiconductive composition.

[0015] The present inventors have appreciated that simple, flexible and cheap heaters can be prepared using a multilayer film structure. Such a film can act as a heater itself or can be used as part of a dedicated heating device as the film is flexible and readily manipulated into a variety of useful heating devices.

[0016] The present inventors have surprisingly observed that high melt flow rate of the polyethylene polymer used in the semiconductive composition in the film plays an important role in the production of multilayer films with uniform thickness and high extrusion output at low pressures. It is very important in a multilayer film that is designed for use as a heater that the film layers have a uniform thickness as the heat generated within the film when current is applied to the film is a function of the film thickness. Thicker films lead to higher temperatures. If therefore a film is produced without a uniform film thickness, this can lead to relative areas which are hotter and colder. From a safety perspective, it is not acceptable that certain parts of an apparently homogeneous heating film might have higher temperatures than other areas. It is therefore important to be able to prepare films with uniform film thickness.

[0017] It is also commercially important that films can be prepared at high extrusion rates and at low pressures. Low pressure extrusion also reduces the risk of film thickness deviation.

[0018] In particular, the inventors have found that when MFR₂₁ is 4.0 g/10min or more, such as 6.0 g/lOmin or more (MFR 21.6; 125°C), more preferably 8.0 g/lOmin or more are of particular utility in this regard. Note that these MFR₂₁ values are determined using ISO1133 at 125°C.

[0019] It was also surprisingly discovered that the semiconductive composition can be co-extruded with a high melting point polyolefin, such as a heterophasic polypropylene copolymer, ideally a random heterophasic polypropylene copolymer or HDPE, to form a second layer in the film (where the semiconductive composition forms a first layer). This gives significant advantages during film extrusion process as the high melting point polyolefin stabilises the electrothermal film, especially in a cast film extrusion process. Without wishing to be limited by theory, it is envisaged that the electrothermal film is electrically isolated from the chill roll, due to the high melting point polyolefin, such as heterophasic polypropylene copolymer or HDPE outer layer. This enables the use of a standard edge pinning system, which is more commonly used for cast film production. The use of this system makes the film extrusion process more stable and improves the film quality and film thickness distribution.

[0020] Moreover, the adhesion between the two layers is such that it is easy to strip off the heterophasic polypropylene copolymer layer to form a monolayer film if the application demands it. The use therefore of a second layer offers the electrothermal advantages above without the need to maintain the second layer in the final film.

Summary of Invention

[0021] Viewed from one aspect the invention provides a film comprising,

a first layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/lOmin or more (measured according to ISO1133 at 125°C under a load of 21.6 kg) and a conductive filler; and

a second layer comprising a polyolefin having a melting point at least 25°C higher than the polyethylene of the electrically semiconductive composition; and

a plurality of conductors in contact with or embedded within the first layer.

[0022] In particular, the invention provides a film comprising first layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/10min or more (measured according to ISO1133 at 125°C under a load of 21.6 kg) and a conductive filler; and

a second layer comprising polypropylene homopolymer, propylene copolymer, or high density polyethylene, especially a heterophasic polypropylene copolymer; and

a plurality of conductors in contact with or embedded within the first layer.

[0023] This second layer also acts as an insulating layer and allows lamination of the first layer to a substrate (such as itself) without the risk of the second layer becoming sticky. This might increase the line speed in an extrusion process.

[0024] Viewed from another aspect the invention provides a film comprising, a first layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising an ethylene alkyl (meth)acrylate or ethylene vinyl acetate polymer having an MFR₂₁ of 4.0 g/10min or more (measured according to ISO1133 at 125°C under a load of 21.6 kg) and a conductive filler; and
a plurality of conductors in contact with or embedded within the first layer.

[0025] Viewed from another aspect the invention provides an electrical heating device comprises a film as hereinbefore defined, e.g. a flat sheet heater.

[0026] Viewed from another aspect the invention provides a process for the preparation of a film comprising the steps of (a)

• providing and melt mixing in an extruder a first composition comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/10min or more (measured at 125°C under a load of 21.6 kg) and a conductive fillerr,
• providing and melt mixing in an extruder, a second electrically semiconductive composition comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/lOmin or more (measured at 125°C under a load of 21.6 kg) and a conductive filler,

(b) applying on a plurality of conductors by coextrusion,

• a meltmix of the first electrically semiconductive composition obtained from step (a),
• a meltmix of the second electrically semiconductive composition obtained from step (a),

to form a film having a core layer comprising a plurality of conductors embedded within and in contact with a first layer formed by said first and second electrically semiconductive compositions.

[0027] Viewed from another aspect the invention provides a process for the prepar/ation of a film comprising the steps of (a)

• providing and melt mixing in an extruder a first composition comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/10min or more (measured at 125°C under a load of 21.6 kg) and a conductive fillerr,
• providing and melt mixing in an extruder, a second electrically semiconductive composition comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/lOmin or more (measured at 125°C under a load of 21.6 kg) and a conductive filler,
• providing and melt mixing in an extruder a polyolefin having a melting point at least 25°C higher than the polyethylene of the electrically semiconductive composition;

(b) applying on a plurality of conductors by coextrusion,

• a meltmix of the first electrically semiconductive composition obtained from step (a),
• a meltmix of the second electrically semiconductive composition obtained from step (a),

and coextruding onto the first or second semiconductive composition a meltmix of the polyolefin having a melting point at least 25°C higher than the polyethylene of the first electrically semiconductive composition,

to form a film having a core layer comprising a plurality of conductors embedded within and in contact with a first layer formed by said first and second electrically semiconductive compositions and a second layer in contact with said first layer comprising said a polyolefin having a melting point at least 25°C higher than the polyethylene of the electrically semiconductive composition.

Brief Description of the Figures

[0028] Figure 1 shows the result of extrusion of EVA1 in the absence of the polypropylene layer. Extreme flow marks are visible on the film.

Detailed Description of Invention

[0029] The present invention relates to a film which can itself act as an electrical heater or can be incorporated into an electrical heating device. The film can be used in a wide variety of objects to provide heat in a safe, cheap and simple manner. The film of the invention uses the principle of positive temperature coefficient (PTC). To avoid overheating and potential destruction of the film, the heat generated in the film is self-limiting and requires no regulating electronics. As temperature increases within the film caused by the power applied to the film, resistance within the semiconductive layer increases until a steady state is reached and no further heating takes place. In one embodiment therefore, the film or heater of the invention contains no regulating electronics, e.g. a heat cut off to prevent overheating.

[0030] The electrically semiconductive composition cannot overheat and requires no overheat protection. The technical solution in this particular invention utilises conversion from electrical to thermal energy by allowing a current to pass through a semiconductive medium with PTC characteristics, which elevates the object temperature above that of its surroundings, until a steady state is reached (self-regulation).

[0031] The film of the invention may be multilayer or monolayer. Conveniently, the film is initially prepared as a multilayer film and the second layer removed should a monolayer film be desired. As explained below, the use of a second layer in the coextrusion process stabilises the first layer during the manufacturing process. This means that the first layer has a more uniform thickness and can also be prepared at higher extrusion rates and at lower pressures than in the absence of this additional layer. The resulting first layer is more homogeneous and contains fewer defects. The second layer may act as an insulation layer and may also act to protect the key first layer from damage. It does not however conduct electricity and hence does not heat up even if the conductors are in contact therewith.

[0032] The electrically semiconductive composition comprises a polyethylene and a conductive filler (e.g. carbon black). The self-regulating thermal phenomenon occurs due to two parallel antagonistic processes:

a. Poor conduction of electrons through the semiconductive medium generates electrical losses, manifested in heat emission.

b. Thermal expansion of the non-conductive part of the material leads to further decrease of the conductivity by separation of the conductive filler particles.

[0033] Once the two processes have equalised, a steady elevated temperature plateau is reached.

[0034] The temperature increase in the electrically semiconductive composition is governed mainly by the distance between the conductors present, the thickness of the electrically semiconductive composition, the amount of conductive filler present and the applied voltage.

[0035] Closer conductors increase the temperature at which a steady elevated temperature plateau is reached.

[0036] A thicker semiconductive layer in the film increases the temperature at which a steady elevated temperature plateau is reached.

[0037] Increases in conductive filler content increases the temperature at which a steady elevated temperature plateau is reached.

[0038] It is preferred that the steady state elevated temperature is no more than 50°C, such as no more than 45°C. The heater should ideally achieve a temperature of at least 30 °C.

[0039] This gives the product designer freedom to alter the heat generated through manipulation of the size and shape of the object, the location of the conductors, the make-up of the semiconductive composition in order to reach a pre-determined target temperature.

First layer - Electrically semiconductive composition

[0040] The film of the invention comprises a first layer comprising, such as consisting of, an electrically semiconductive composition. The first layer is therefore a semiconductive layer. The electrically semiconductive composition comprises a polyethylene. It preferred if the polyethylene is one prepared in a high temperature autoclave or tubular process such as a LDPE homopolymer or copolymer.

[0041] Although the term LDPE is an abbreviation for low density polyethylene, the term is understood not to limit the density range, but covers the LDPE-like high pressure (HP) polyethylenes. The term LDPE describes and distinguishes only the nature of HP polyethylene with typical features, such as different branching architecture, compared to the polyethylene produced in the presence of an olefin polymerisation catalyst.

[0042] The LDPE as said polyolefin means a low density homopolymer of ethylene (referred herein as LDPE homopolymer) or a low density copolymer of ethylene with one or more comonomer(s) (referred herein as LDPE copolymer).

[0043] It is preferred if the electrically semiconductive composition comprises an LDPE copolymer. The one or more comonomers of LDPE copolymer are preferably selected from the polar comonomer(s), non-polar comonomer(s) or from a mixture of the polar comonomer(s) and non-polar comonomer(s). Moreover, said LDPE homopolymer or LDPE copolymer may optionally be unsaturated.

[0044] As a polar comonomer for the LDPE copolymer, comonomer(s) containing carboxyl and/or ester group(s) are used as said polar comonomer. Still more preferably, the polar comonomer(s) of LDPE copolymer is selected from the groups of acrylate(s), methacrylate(s) or acetate(s), or any mixtures thereof.

[0045] If present in said LDPE copolymer, the polar comonomer(s) is preferably selected from the group of alkyl acrylates, alkyl methacrylates or vinyl acetate, or a mixture thereof. The use of ethylene alkyl acylates or ethylene vinyl acetate is preferred.

[0046] Further preferably, said polar comonomers are selected from C₁- to C₆-alkyl acrylates, C₁- to C₆-alkyl methacrylates or vinyl acetate. Still more preferably, said LDPE copolymer is a copolymer of ethylene with C₁- to C₄-alkyl acrylate, such as methyl, ethyl, propyl or butyl acrylate, or vinyl acetate, or any mixture thereof. The use of ethylene methyl acrylate (EMA), ethylene ethyl acrylate (EEA), ethylene butyl acrylate (EBA) or ethylene vinyl acetate (EVA) is preferred.

[0047] As the non-polar comonomer(s) for the LDPE copolymer preferred options are polyunsaturated comonomers comprising C and H atoms only. In a preferred embodiment, the polyunsaturated comonomer consists of a straight carbon chain with at least 8 carbon atoms and at least 4 carbon atoms between the non-conjugated double bonds, of which at least one is terminal.

[0048] A preferred diene compound is 1,7-octadiene, 1,9-decadiene, 1,11-dodecadiene, 1,13-tetradecadiene, or mixtures thereof. Furthermore, dienes like 7-methyl-1,6-octadiene, 9-methyl-1,8-decadiene, or mixtures thereof can be mentioned.

[0049] If the LDPE polymer is a copolymer, it preferably comprises 1.0 to 40 wt.-%, more preferably 5.0 to 35 wt.-%, still more preferably 10 to 30 wt%%, of one or more comonomer(s).

[0050] Where there is a polar comonomer, the comonomer content is preferably 5.0 to 30 wt%, such as 7.5 to 20 wt% in the polymer. The use of ethylene methyl acrylate (EMA), ethylene ethyl acrylate (EEA), ethylene butyl acrylate (EBA) or ethylene vinyl acetate (EVA) is preferred especially where there is 5 to 30 wt%, such as 7.5 to 20 wt% comonomer present.

[0051] The melting point of the polyethylene may be 90 to 120°C.

[0052] A key aspect of the invention is the MFR of the polyethylene when measured under a load of 21.6 kg/125°C.

[0053] MFR₂₁ values of 4.0 g/lOmin or more are required, such as at least 6.0 g/10 min, even more preferably 8.0 to 15 g/10 min, and most preferably at least 10.0 g/10 min. An upper limit of 25 g/10min is preferred, such as 18 g/10min. Using higher MFR values appears to give more uniform layer thicknesses and allows higher extrusion output without changing the ability of the polyethylene to act as a heater through PTC principles.

[0054] Any LDPE homopolymer or copolymer may have a density of 905 to 935 kg/m³, such as 910 to 925 kg/m³.

[0055] The polyethylene can be produced by any conventional polymerisation process. Preferably, it is an LDPE and is produced by radical polymerisation, such as high pressure radical polymerisation. High pressure polymerisation can be effected in a tubular reactor or an autoclave reactor. Preferably, it is a tubular reactor. In general, the pressure can be within the range of 1200-3500 bars and the temperature can be within the range of 150°C-350°C. Further details about high pressure radical polymerisation are given in WO93/08222, which is herewith incorporated by reference. The polymers of the semiconductive composition are well known and are commercially available.

[0056] The electrically semiconductive composition may comprise at least 50 wt% of the polyethylene, such as at least 60 wt%. Any layer in which the electrically semiconductive composition is present may consist of the electrically semiconductive composition. Thus, any layer in which the electrically semiconductive composition is present may comprise at least 50 wt% of the polyethylene, such as at least 60 wt%. The polyethylene will form the balance of the electrically semiconductive composition once all other components are determined. Ideally, the polyethylene is the only polymer component present in the semiconductive composition.

Conductive Filler

[0057] According to the present invention, the electrically semiconductive composition further comprises a conductive filler such as carbon black.

[0058] Suitable conductive fillers include graphite, graphene, carbon fibres, carbon nanotubes, metal powders, metal strands or carbon black. The use of carbon black is preferred.

[0059] The semiconductive properties result from the conductive filler added. Thus, the amount of conductive filler is at least such that a semiconducting composition is obtained. Depending on the desired use and conductivity of the composition, the amount of conductive filler can vary. Preferably, the electrically semiconductive composition comprises 5-50 wt% conductive filler, such as 15 to 50 wt%. In other preferred embodiments, the amount of conductive filler is 5-48 wt.-%, 10-45 wt%, 20-45 wt%, 25-45 wt% or 30-41 wt%, based on the weight of the electrically semiconductive composition.

[0060] It also follows that the first layer may comprise 5-50 wt% conductive filler, such as 15 to 50 wt%. In other preferred embodiments, the amount of conductive filler in the first layer is 5-48 wt.%, 10-45 wt%, 20-45 wt%, 25-45 wt% or 30-41 wt%, based on the weight of the layer.

[0061] Any carbon black can be used which is electrically conductive. Examples of suitable carbon blacks include furnace blacks, channel blacks, gas blacks, lamp blacks, thermal blacks and acetylene blacks. Additionally, graphitised furnace blacks (as produced by Imerys) and high structure blacks (known as Ketjenblacks produced by Nouryon) may also be used. Mixtures may also be used. Where a blend of carbon blacks is used then this percentage refers to the sum of the carbon blacks present.

[0062] The carbon black may have a nitrogen surface area (BET) of 5 to 1500 m²/g, for example of 10 to 300 m²/g, e.g. of 30 to 200 m²/g, when determined according to ASTM D3037-93. Further, the carbon black may have one or more of the following properties:

i) a primary particle size of at least 5 nm which is defined as the number average particle diameter according to ASTM D3849-95a,

ii) iodine adsorption number (IAN) of at least 10mg/g, for example 10 to 300 mg/g, e.g. 30 to 200 mg/g, when determined according to ASTM D-1510; and/or

iii) DBP (dibutyl phthalate) absorption number (= oil absorption number) of at least 30 cm³/100g, for example 60 to 300 cm³/100g, e.g. 70 to 250 cm³/100g, for example 80 to 200 cm³/100g, e.g. 90 to 180 cm³/100g, when measured according to ASTM D 2414.

[0063] Furthermore, the carbon black may have one or more of the following properties:

a) a primary particle size of at least 15 nm which is defined as the number average particle diameter according ASTM D3849-95a;

b) iodine number of at least 30 mg/g according to ASTM D1510;

c) oil absorption number of at least 30 ml/100g which is measured according to ASTM D2414.

[0064] Furnace carbon blacks are preferred. This is a generally acknowledged term for the well-known carbon black type that is produced continuously in a furnace-type reactor. As examples of carbon blacks, the preparation process thereof and the reactors, reference can be made to i.a. EP-A-0629222 of Cabot, US 4,391,789, US 3,922,335 and US 3,401,020. As an example of commercial furnace carbon black grades described in ASTM D 1765-98b i.a. N351, N293 and N550, can be mentioned.

Other components

[0065] The semiconductive composition may be crosslinked using peroxide or silane moisture curing systems. Crosslinking may also be effected using irradiation to avoid the need for a crosslinking agent.

[0066] Preferably, crosslinking is avoided and the resulting film is a more recyclable product. The semiconductive composition of the invention is preferably not crosslinked.

Antioxidant

[0067] The semiconductive composition may contain an antioxidant. As antioxidant, sterically hindered or semi-hindered phenols, aromatic amines, aliphatic sterically hindered amines, organic phosphates, thio compounds, polymerized 2,2,4-trimethyl-1,2-dihydroquinoline and mixtures thereof, can be mentioned.

[0068] More preferred, the antioxidant is selected from the group of 4,4'-bis(1,1'dimethylbenzyl)diphenylamine, para-oriented styrenated diphenylamines, 4,4'-thiobis (2-tert. butyl-5-methylphenol), polymerized 2,2,4-trimethyl-1,2-dihydroquinoline, 4-(1-methyl-1-phenylethyl)N-[4-(1-methyl-1-phenylethyl)phenyl] aniline or derivatives thereof.

[0069] More preferred, the antioxidant is selected from the group (but not limited to) of 4,4'- bis(1,1'dimethylbenzyl)diphenylamine, para-oriented styrenated diphenylamines, 4,4'-thiobis (2-tert. butyl-5-methylphenol), 2,2'-thiobis(6-t-butyl-4-methylphenol), distearylthiodipropionate, 2,2'-thio-diethyl-bis-(3-(3,5-di-tertbutyl-4-hydroxyphenyl)propionate, polymerized 2,2,4-trimethyl-1,2-dihydroquinoline, or derivatives thereof. Of course, not only one of the above-described antioxidants may be used but also any mixture thereof.

[0070] The amount of antioxidant, optionally a mixture of two or more antioxidants, can range from 0.005 to 2.5 wt-%, such as 0.01 to 2.5 wt-%, preferably 0.01 to 2.0 wt-%, more preferably 0.03 to 2.0 wt-%, especially 0.03 to 1.5 wt-%, more especially 0.05 to 1.5 wt%, or 0.1 to 1.5 wt% based on the weight of the semiconductive composition.

[0071] The semiconductive composition may comprise further additives. As possible additives stabilisers, processing aids, flame retardant additives, acid scavengers, inorganic fillers, voltage stabilizers, or mixtures thereof can be mentioned.

[0072] Preferably, the electrically semiconductive composition has a volume resistivity, measured at 40°C, of less than 20 Ohm • cm. Preferably, the electrically semiconductive composition has a volume resistivity, measured at 25°C, of less than 12 Ohm • cm.

[0073] The first layer may have a thickness of 50 to 3000 µm, such as 75 to 2000 µm, especially 100 to 1000 µm.

Second layer

[0074] The film of the invention may be multilayer or monolayer. It is preferred if the film is prepared as a multilayer film and if desired, the second layer is removed before installation in a heating device. It is therefore preferred therefore if the film comprises a second layer. In any monolayer film, the layer is as defined above for the first layer.

[0075] The second layer preferably comprises a polyolefin which has a melting point at least 25°C higher than the polyethylene in the electrically semiconductive composition. Typically, the polyethylene of the electrically semiconductive composition will have a melting point of no more than 120°C. It is therefore preferred if the polyolefin of the second layer has a melting point of at least 145°C, preferably at least 155°C. The upper limit might be 190°C. Importantly, this polyolefin has sufficiently high melting point that it is possible to laminate the film above the melting temperature of the polyethylene of the electrically semiconductive composition without the second layer becoming sticky.

[0076] It is preferred if the second layer consisting essentially of the polyolefin, i.e. the polyolefin is the only component present other than any standard polymer additives.

[0077] It is preferred if the polyolefin of the second layer is a polypropylene homopolymer or a polypropylene copolymer, preferably a polypropylene copolymer or a high density polyethylene.

[0078] It is most preferred if the polypropylene copolymer is a heterophasic polypropylene copolymer or a high density polyethylene polymer. It is preferred if the second layer comprises a heterophasic polypropylene copolymer, especially a random heterophasic polypropylene copolymer. The second layer preferably comprises at least 80 wt% of the a heterophasic polypropylene copolymer or a high density polyethylene polymer, such as at least 90 wt%. In one embodiment the second layer consists essentially of the heterophasic polypropylene copolymer or a high density polyethylene polymer, i.e. the only other components that may be present are standard polymer additives.

[0079] The heterophasic polypropylene copolymer is one that comprises a matrix component, typically a propylene homopolymer or propylene copolymer with low comonomer content and an amorphous component, typically a propylene copolymer with high comonomer content. These polymers, often called RaHeCos are well known in the art and can be purchased from commercial suppliers, e.g. BC918CF
It is preferred if the heterophasic copolymer comprises ethylene as the comonomer. It is preferred if the heterophasic copolymer has an MFR₂ of 0.5 to 10 g/lOmin (measured using ISO1133 at 230°C).

[0080] A preferred heterophasic propylene ethylene copolymer has an MFR₂ of 0.5 to 10 g/lOmin and comprises:

(i) 50 to 90 wt% of a propylene homopolymer or propylene ethylene copolymer matrix having up to 4 wt% ethylene; and

(ii) 10 to 50 wt% of an ethylene propylene rubber (EPR) dispersed in the matrix.

[0081] It is preferred if the heterophasic propylene ethylene copolymer has a xylene cold soluble content (XS) of 12 to 50 %.

[0082] It is preferred if the ethylene content of the xylene cold soluble fraction of said heterophasic propylene ethylene copolymer is between 18 and 70 wt.%. Alternatively viewed, it is preferred if component (ii) has an ethylene content of 18 to 70 wt%.

[0083] If the second layer comprises an HDPE, this preferably has a density of 940 to 970 kg/m³, such as 945 to 965 kg/m³. It preferably has a MFR₂ of 0.1 to 10 g/lOmin, such as 0.5 to 5.0 g/lOmin.

[0084] It is preferred if the second layer is free of conductive filler, e.g. carbon black. The second layer may therefore effectively act as an insulating layer.

[0085] The second layer preferably has a thickness of 10 to 200 µm, such as 25 to 150 µm, especially 25 to 100 µm.

Conductors

[0086] The film of the invention comprises a plurality of conductors. The term plurality is used herein to imply at least 2, preferably at least 4 conductors. Ideally, the film of the invention comprises an even number of conductors. In use, the conductors have alternate polarity.

[0087] The conductors can be made from any suitable conductive metal, typically copper or aluminium. The conductor may be in the form of a tape, foil or wire. Conductors may have a diameter or thickness of 1.0 µm to 2.0 mm, such as 5.0 µm to 1.0 mm. Conductors can be spherical in cross-section in which case the diameter is given above. Some conductors may have a width of 0.5 to 15 mm, such as 1.0 to 10 mm. The length of the conductor is governed by the size of the heater into which the film will be incorporated. The conductor should pass through the majority, such as the whole, of the film. The conductors are elongate which implies that the conductors are long and thin compared to their width and thickness. The conductors are often wires or tapes, which can be made via well-known processes such as extrusion.

[0088] Each conductor may be provided with an electrode to allow the plurality of conductors to be interconnected and to allow the application of an external power source to create a circuit and hence heat. The conductors may be designed to be directly solderable for ease of installation.

[0089] The film may comprise a minimum of 2 separate conductors, such as at least but it may contain many more conductors. The conductors are spaced apart from each and hence do not touch. The conductors are preferably substantially parallel to each other. All conductors should preferably be evenly spaced from each other to ensure an even temperature on application of power. By evenly spaced means that the distance between adjacent conductors is always the same. The conductors are preferably linear. In theory however the conductors might be curved (SS shaped for example) such that they remain equidistant from each other at all times. We regard this as being "parallel".

[0090] In one embodiment, the gap between the conductors is 20 to 150 mm, preferably 30 to 90 mm, such as 40 to 80 mm.

[0091] It is preferred therefore if the film comprises a plurality of conductors that are evenly spaced apart from and substantially parallel to each other, e.g. wherein the distance between conductors is 20 to 150 mm.

[0092] In one embodiment the conductors are in direct contact with the electrically semiconductive composition. In this embodiment therefore there should not be a layer separating the electrically semiconductive composition from the conductor.

[0093] Alternatively, an adhesive layer may be provided on at least a part the conductors. In effect therefore the conductors may be coated (at least partially) with an adhesive layer. This adhesive layer partially or completely covers the conductor and not only adheres to the semiconductive composition but provides an electrical contact with the semiconductive composition.

[0094] Suitable adhesives are conductive adhesives such as those comprising silicone or epoxy resins filled with metallics or conductive carbon fragments.

[0095] The conductors may be attached to the outside of the semiconductive layer or be embedded within the electrically semiconductive layer. The latter option is preferred. This can be achieved if the conductors are coextruded with two preferably identical layers of the electrically semi-conductive composition. It might also be achieved by sandwiching the conductors between two layers of electrically semiconductive composition, e.g. using well known lamination techniques. If two layers of identical semiconductive composition are co-extruded or laminated together, we regard the resulting structure as containing one layer, i.e. the first layer as defined herein.

[0096] It is preferred therefore if the plurality of conductors are embedded within the first layer, e.g. wherein the first layer is provided as two separate sub layers such that said plurality of conductors are sandwiched between and in contact with said sub layers.

[0097] Where the conductors are coextruded with the first layer, it is preferred that the conductors are a parallel with the machine direction of the film. It is however possible that the conductors are oriented in the transverse direction in particular if these are retrofitted to the film after formation or if the first layer is prepared by lamination.

Production

[0098] It is possible for the claimed film to be prepared by colamination. In such a process, two electrically semiconductive layers can be prepared, e.g. via extrusion. These may be allowed to cool before colamination occurs. The layers may be the same or different. Ideally they are the same. These layers are preferably the same thickness.

[0099] These layers can then be used to sandwich a layer of the conductors. One layer is therefore placed above and one layer placed below the conductors and the ensemble compressed together. The gaps between the conductors are therefore filled by the electrically semiconductive layers. It will be appreciated that the conductor layer is often very thin compared to the electrically semiconductive layers.

[0100] It is possible to heat one surface of one or both electrically semiconductive layers before colamination such that when colaminated together, the electrically semiconductive layers adhere to the conductors and adhere to the other electrically semiconductive layer without the use of a separate adhesive. In this way, the conductor layer becomes embedded within an electrically semiconductive composition.

[0101] Alternatively an adhesive is used to help stick the conductors to the semiconductive composition. The adhesive can be coated onto the conductors before contact with the semiconductive composition is made.

[0102] The film of the invention can therefore be prepared continuously.

[0103] Importantly, the semiconductive composition can be extruded onto the conductors and hence these are embedded within the semiconductive composition during the extrusion process rather than separately adhered to the semiconductive composition. The semiconductive composition that forms the layer above and below the plurality of conductors can therefore be extruded continuously onto those conductors.

[0104] The process described herein is therefore one that can be operated continuously maximising the value of the formed product. The film of the invention is cheap. It is also thin and flexible.

[0105] It is preferred therefore if two semi-conductive layers are coextruded to encompass the central conductors.

[0106] In the preferred embodiment of the invention the process comprises the steps of (a)

• providing and melt mixing in an extruder, a first electrically semiconductive composition as herein defined,
• providing and melt mixing in an extruder, a second electrically semiconductive composition as herein defined,
  (b) applying on a plurality of conductors by coextrusion,
• a meltmix of the first electrically semiconductive composition obtained from step (a),
• a meltmix of the second electrically semiconductive composition obtained from step (a),
  to form a film having a core layer comprising a plurality of parallel evenly spaced apart conductors embedded within and in contact a first layer formed from said first and second electrically semiconductive compositions.

[0107] This process can be readily adapted to include further layers above or below the semiconductive layers. In particular, the second layer as defined herein can also be coextruded above or below the electrically semiconductive compositions. Alternatively, the second layer can be laminated above or below the electrically semiconductive compositions. Thus, after production of a first film, a second layer comprising a polyolefin with a melting point at least 25°C higher than the melting point of the polyethylene of the semiconductive layer, such as a polypropylene homopolymer or copolymer, especially a polypropylene copolymer or high density polyethylene, preferably a heterophasic polypropylene copolymer is co-extruded or co-laminated outside said first or second semi-conductive layer.

[0108] Most preferably, the process for the preparation of a film comprises the steps of (a)

• providing and melt mixing in an extruder a first composition comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/10min or more (measured at 125°C under a load of 21.6 kg) and a conductive fillerr,
• providing and melt mixing in an extruder, a second electrically semiconductive composition comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/lOmin or more (measured at 125°C under a load of 21.6 kg) and a conductive filler,
• providing and melt mixing in an extruder a polyolefin having a melting point at least 25°C higher than the polyethylene of the electrically semiconductive composition;

(b) applying on a plurality of conductors by coextrusion,

• a meltmix of the first electrically semiconductive composition obtained from step (a),
• a meltmix of the second electrically semiconductive composition obtained from step (a),

  and coextruding onto the first or second semiconductive composition a meltmix of the polyolefin having a melting point at least 25°C higher than the polyethylene of the first electrically semiconductive composition,

  to form a film having a core layer comprising a plurality of conductors embedded within and in contact with a first layer formed by said first and second electrically semiconductive compositions and a second layer in contact with said first layer comprising said a polyolefin having a melting point at least 25°C higher than the polyethylene of the electrically semiconductive composition.

[0109] The films may be effected using cast or blown film extrusion. In cast film extrusion, a slit die is employed positioned vertically so as to extrude a fine melt film onto a highly polished, high speed chill roll. The melt is pinned to the surface of the chill roll by either the pressure from an air knife or a vacuum box located close to the roll. This causes the fine film to be rapidly quenched, which improves its mechanical properties and clarity. The film then travels through a further series of chill, polishing and nip rolls, which help to draw the film down to the correct thickness, before its edges are trimmed and it is wound onto a drum for storage. By using a second layer as defined herein, edge pinning is possible during the film manufacturing process. During an edge pinning process, a charging applicator at each edge of the film on the chill roll applies a static charge to the extruded film as it contacts the chill roll. The static charge effectively prevents 'neck-in' of the film.

[0110] In blown film extrusion, the molten plastic from the extruder passes through an annular die and emerges as a thin tube. A supply of air to the inside of the tube prevents it from collapsing and may be used to inflate it to a larger diameter. Initially the bubble consists of molten plastic but a jet of air around the outside of the tube (cooling ring) promotes cooling and at a certain distance from the die exit, a freeze line can be identified. Eventually the cooled film passes through collapsing guides and nip rolls before being taken off to storage drums or, for example, gussetted and cut to length.

[0111] In one embodiment, crosslinking conditions can then be applied to cause a crosslinking reaction. It is preferred however if no crosslinking reaction is used.

[0112] Melt mixing means mixing above the melting point of at least the major polymer component(s) of the mixture and is typically carried out in a temperature of at least 10-15°C above the melting or softening point of polymer component(s).

[0113] The term coextrusion means herein that two or more layers are extruded in the same extrusion step. The term coextrusion means that all or part of the layer(s) are formed simultaneously using one or more extrusion heads.

[0114] The film of the invention can therefore be seen as a multilayer film comprising, in this order, a first layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene and a conductive filler;
a second layer comprising a polyolefin, such as a polypropylene copolymer, especially a heterophasic polypropylene copolymer or HDPE as herein defined and a conductor layer comprising a plurality of spaced apart conductors embedded within the first layer or in contact with the first layer.

Flat sheet Heater

[0115] The film of the invention is flexible and can therefore be used directly as a flat sheet heater or can be incorporated into a heating device. The film of the invention may be provided with one or more additional layers to protect the semiconductive composition from damage. For example, an aesthetic top layer can be textile fabric, non-woven or solid sheet (rubber, plastic, paper, wood, metal, etc.). Optionally, no top layer(s) are used.

[0116] The top layer may be extrudable, e.g. a polyolefin layer.

[0117] In a preferred embodiment the film is provided with an insulation layer or heat reflective layer at the base of the heater. Such an insulation layer may be electrically insulating, thermally insulating or both. Such a layer increases the heating effectiveness of the film. Such a layer may comprise a polyolefin such as a polyethylene, especially an LDPE, e.g. an LDPE homopolymer. Preferred insulation layers use LDPE as the only polymer component. Such a layer is preferably one that can be coextruded although lamination of this layer is also an option.

[0118] The film may be provided with a support to provide mechanical strength to the film.

[0119] In use current is applied to the film via the conductors to generate heat. Typically voltages are 10 to 40 v, such as 12 to 30 v. the film can be heated therefore using batteries or via the mains with suitable transformer. The application of the power to the film leads to almost instant heat. There is no risk of electrocution as the voltage used does not need to be high.

[0120] The film can be prepared in any desired dimensions. The width of the heater can be adjusted readily to any possible use. The width may be a function of the coextrusion apparatus and sheets from 5 cm to 5 metres can be produced readily.

[0121] As previously discussed, the heating power of the film can be controlled through the thickness of the sheet, separation of the conductors, conductive filler content and applied voltage. Moving the conductors closer together increases the wattage and hence the heat generated. The relationship can be expressed as power = [voltage]²/resistance.

[0122] Thicker films tend to increase the power output.

Applications

[0123] The film of the invention can be utilised in many fields. Applications of the technology described herein are therefore widespread.

[0124] We often provide thermal comfort in winter by heating the entire volume of air in a room or building. In earlier times, our ancestor's concept of heating was more localized: heating people, not places. They used radiant heat sources that warmed only certain parts of a room, creating micro-climates of comfort. These people countered the large temperature differences with insulating furniture, such as hooded chairs and folding screens, and they made use of additional, personal heating sources that warmed specific body parts. It would make a lot of sense to restore this old way of warming, especially since modern technology has made it so much more practical, safe and efficient.

[0125] The film of the invention may therefore be employed within an item of furniture such as a screen, chair or sofa.

[0126] In one embodiment the film of the invention might be used in a heated garment. Heated garments available today have small wires (often made of brittle carbon fibres) built into them. They heat up when a low voltage electric current is passed through. There are two main types of heated clothing, battery powered or powered by a vehicle (e.g. heated gloves on a motorcycle). The films of the invention are ideally suited for use in both these applications.

[0127] The film may also be used in a blanket. A major concern with electric heating blankets on the market today is fire risk. These blankets tend to overheat. Using the film of the present invention that risk is eliminated.

[0128] Radiators are large, immobile and often unattractive. In many parts of the world, radiators are hidden behind more aesthetically pleasing covers of various designs. These covers may also reduce noise or protect against the touching of radiators that get excessively hot. But hiding the radiator is not efficient because adding a radiator cover slows the movement of heat out of the radiator and into the room. The rate of heat loss out through the building's exterior wall is likely to be increased.

[0129] The films of the invention can replace radiators or be used in walls, under floors, in ceilings as heaters. The films could even be included within a carpet or rug or other floor covering.

[0130] Electric cars generate next to no heat as opposed to conventional passenger vehicles, which produce more than enough engine heat to heat the interior. An additional electric heater is therefore required in an electric vehicle to heat the interior.

[0131] This heater is supplied with power by the same battery that provides the engine with energy. This can reduce the maximum possible drive distance by a considerable amount.

[0132] Thus, there is a need for heating e-vehicles as efficiently as possible. The present invention might be used to heat inner contact surfaces such as steering wheel, armrest, door panels, seats within the vehicle. More efficient heating can be envisaged compared to heating the entire inner volume of the car, especially for short journeys.

[0133] The film of the invention could be used to prevent ice or snow build up on a critical surface such as a solar panel. Films might therefore have utility in deicing operations. Other surfaces might be wing mirrors.

[0134] The films are flexible and might be wrapped around pipes to prevent liquid freezing therein. Films can furthermore be used to keep fluids heated e.g. in swimming pools or liquid containers. The skilled person can device many applications of these versatile heating films.

Melt Flow Rate

[0135] The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer. The MFR is determined at 230°C for polypropylene and may be determined at different loadings such as 2.16 kg (MFR2) or 21.6 kg (MFR21). The MFR is determined at 190°C for the HDPE and may be determined at different loadings such as 2.16 kg (MFR2) or 21.6 kg (MFR21).

[0136] The MFR is determined at 125°C for polyethylene present within the semiconductive composition. It may be determined at loading of 21.6 kg (MFR21).

Determination of xylene soluble fraction (XS):

[0137] The xylene soluble fraction (XS) as defined and described in the present invention is determined in line with ISO 16152 as follows: 2.0 g of the polymer were dissolved in 250 ml p-xylene at 135 °C under agitation. After 30 minutes, the solution was allowed to cool for 15 minutes at ambient temperature and then allowed to settle for 30 minutes at 25 ± 0.5 °C. The solution was filtered with filter paper into two 100 ml flasks. The solution from the first 100 ml vessel was evaporated in nitrogen flow and the residue dried under vacuum at 90 °C until constant weight is reached. The xylene soluble fraction (percent) can then be determined as follows:

wherein m0 designates the initial polymer amount (grams), m1 defines the weight of residue (grams), v0 defines the initial volume (milliliter) and v1 defines the volume of the analysed sample (milliliter).

Melting Points

[0138] Melting temperature Tm, can be measured with Mettler TA820 differential scanning calorimetry (DSC) on 3±0.5 mg samples. Melting curves were obtained during 10°C/min cooling and heating scans between 30°C and 225°C. Melting temperatures were taken as the peaks of endotherms and exotherms.

Examples

[0139] The following materials are used herein:

Semiconductive composition EVA1- Ethylene vinyl acetate copolymer with 17 wt% comonomer and a density of 920 kg/m³ and combined with carbon black (38 wt%) and having an MFR₂₁ of 7.0 g/lOmin measured at 125°C, melting point less than 120°C.

Semiconductive composition EVA2- Ethylene vinyl acetate copolymer with 17 wt% comonomer and a density of 920 kg/m³ and combined with carbon black (38 wt%) and having an MFR₂₁ of 11.0 g/lOmin measured at 125°C, melting point less than 120°C.

BC918CF: density 905 kg/m³ and MFR₂ of 3.0 g/lOmin (Commercial polymer from Borealis AG - PP), melting point 168°C.

Example 1 - monolayer film

[0140] A monolayer layer film was prepared using a SML cast film co-extrusion line. The 3-layer line is equipped with an 1200 mm Cloeren and a flexible die gap of 0,5 - 1,0 mm. The film thickness is adjusted automatically by using a radioactive thickness measurement device in combination with automatic controlled heating bolts at the die. The chill roll temperature was set to 15 °C. The film was wounded on 3" cores.

[0141] EVA1 was extruded in layers 1 to 3 to form a monolayer. The overall film thickness was 125 µm.

Example 2 - multilayer film

[0142] A two layer co-extruded film was prepared using a SML cast film co-extrusion line. The 3-layer line is equipped with an 1200 mm Cloeren and a flexible die gap of 0,5 - 1,0 mm. The film thickness is adjusted automatically by using a radioactive thickness measurement device in combination with automatic controlled heating bolts at the die. The chill roll temperature was set to 15 °C. The film was wounded on 3" cores.

[0143] EVA2 was extruded in layers 1 and 2 to form a first layer and BC918CF extruded in layer 3 to form the second layer in a two layer film. The overall film thickness was 125 µm with a 50 µm second layer.

Results

Mono - 125 µm EVA

EVA1 - Low MFR

[0144] 

EX A - throughput:	149 kg/h
EX A - torque:	76,9 %
EX A - MP:	242 / 206 bar (before / after filter)
EX A - MT:	193 / 186 °C (before / after filter)

MP=melt pressure MT = melt temperature

Multilayer film - 125 µm EVA layer + 50 µm PP layer

EVA2 - High MFR

[0145] 

Total - throughput:	201,2 kg/h
Material:	EVA2
EX A - throughput:	137,1 kg/h
EX A - torque:	56,3 %
EX A - MP:	213 / 172 bar	(before / after filter)
EX A - MT:	218 / 216 °C	(before / after filter)
Material:	EVA2
EX B - throughput:	30,3 kg/h
EX B - torque:	26,5 %
EX B - MP:	209 / 195 bar	(before / after filter)
EX B - MT:	216 / 211 °C	(before / after filter)
Material:	PP
EX C - throughput:	33,8 kg/h
EX C - torque:	25,0 %
EX C - MP:	205 / 137 bar	(before / after filter)
EX C - MT:	243 / 241 °C	(before / after filter)

Discussion

[0146] Producing films with EVA1 showed an extreme variation in throughput and melt pressure level. The reported values for EVA1 are a snapshot of the variation, but it shows an increased pressure level compared to EVA2. The pressure variation is evidenced in figure 1 via extreme flow marks. Usually the film thickness variation is measured, but this was not done for EVA1, because the film quality was such bad that the measurement didn't work.

[0147] The data suggests that the film comprising the higher MFR semiconductive composition has a more uniform thickness and high extrusion output at low pressures.

[0148] In the absence of the heterophasic polypropylene copolymer layer, i.e. if there is no insulation between the chill roll and the film, edge pinning is not possible.

Example 3 - multilayer film

[0149] A two layer co-extruded film was prepared using a SML cast film co-extrusion line. The 3-layer line is equipped with an 1200 mm Cloeren and a flexible die gap of 0,5 - 1,0 mm. The film thickness is adjusted automatically by using a radioactive thickness measurement device in combination with automatic controlled heating bolts at the die. The chill roll temperature was set to 15 °C. The film was wounded on 3" cores.

[0150] EVA2 was extruded in layers 1 and 2 to form a first layer and BC918CF extruded in layer 3 to form the second layer in a two layer film. The overall film thickness was 300 µm with a 50 µm second layer.

Example 4 - multilayer film

[0151] A two layer co-extruded film was prepared using a SML cast film co-extrusion line. The 3-layer line is equipped with an 1200 mm Cloeren and a flexible die gap of 0,5 - 1,0 mm. The film thickness is adjusted automatically by using a radioactive thickness measurement device in combination with automatic controlled heating bolts at the die. The chill roll temperature was set to 15 °C. The film was wounded on 3" cores.

[0152] EVA2 was extruded in layers 1 and 2 to form a first layer and BC918CF extruded in layer 3 to form the second layer in a two layer film. The overall film thickness was 550 µm with a 50 µm second layer.

Claims

1. A film comprising,

a first layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/lOmin or more (measured according to ISO1133 at 125°C under a load of 21.6 kg) and a conductive filler; and

a second layer comprising a polyolefin with a melting point at least 25°C higher than the melting point of the polyethylene of the electrically semiconductive composition, such as a polypropylene homopolymer, a polypropylene copolymer, or a high density polyethylene, preferably a heterophasic polypropylene copolymer; and

a plurality of conductors in contact with or embedded within the first layer.

2. A film, such as a monolayer film, comprising,

a first layer comprising an electrically semiconductive composition with a positive temperature coefficient comprising an ethylene alkyl (meth)acrylate or ethylene vinyl acetate polymer having an MFR₂₁ of 4.0 g/lOmin or more (measured according to ISO1133 at 125°C under a load of 21.6 kg) and a conductive filler; and

a plurality of conductors in contact with or embedded within the first layer.

3. A film as claimed in any preceding claim wherein the plurality of elongate conductors are embedded within the first layer, e.g. wherein the first layer is provided as two separate sub layers such that said plurality of elongate conductors are sandwiched between and in contact with said sub layers.

4. A film as claimed in any preceding claim wherein the first layer, through its manufacturing, has a machine direction and said plurality of elongate conductors are parallel with the machine direction of the first layer.

5. A film as claimed in any preceding claim wherein the plurality of elongate conductors are evenly spaced apart from and substantially parallel to each other, e.g. wherein the elongate conductors are spaced apart by a distance of between 20 to 150 mm.

6. A film as claimed in any preceding claim wherein the electrically semi-conductive composition is not crosslinked or is crosslinked, e,g. using a peroxide crosslinking agent.

7. A film as claimed in any preceding claim wherein the polyethylene has an MFR₂₁ of 6.0 g/lOmin or more, such as 8.0 to 18 g/lOmin (measured according to ISO1133 at 125°C under a load of 21.6 kg).

8. A film as claimed in any preceding claim wherein the electrically semiconductive composition comprises an ethylene alkyl (meth)acrylate or ethylene vinyl acetate polymer, especially an ethylene vinyl acetate polymer.

9. A film as claimed in any preceding claim wherein the conductive filler comprises carbon black.

10. A film as claimed in any preceding claim wherein the electrically semiconductive composition comprises 15-50 wt% conductive filler, based on the total weight of the electrically semiconductive composition.

11. A film as claimed in any preceding claim wherein the second layer is free of conductive filler.

12. A film as claimed in any preceding claim wherein the first layer has a thickness of 50 to 3000 µm, such as 75 to 2000 µm, especially 100 to 1000 µm; and/or
wherein the second layer has a thickness of 10 to 200 µm, such as 25 to 150 µm, especially 25 to 100 µm.

13. An electrical heating device comprising the film of claims 1 to 12.

14. A multilayer flat sheet electrical heater as claimed in claim 13 obtainable by coextrusion comprising, in this order, a layer (Ai) comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/lOmin or more (measured according to ISO1133 at 125°C under a load of 21.6 kg) and a conductive filler; and

a conductor layer comprising a plurality of elongate conductors;

a first layer (Aii) comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/lOmin or more (measured according to ISO1133 at 125°C under a load of 21.6 kg) and a conductive filler; and

a second layer comprising a polyolefin with a melting point at least 25°C higher than the melting point of the polyethylene of the electrically semiconductive composition, such as a polypropylene homopolymer, a polypropylene copolymer or a high density polyethylene, preferably a heterophasic polypropylene copolymer.

15. The process for the preparation of a film as claimed in claim 1 comprising the steps of (a)

- providing and melt mixing in an extruder a first composition comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/10min or more (measured at 125°C under a load of 21.6 kg) and a conductive fillerr,

- providing and melt mixing in an extruder, a second electrically semiconductive composition comprising an electrically semiconductive composition with a positive temperature coefficient comprising a polyethylene having an MFR₂₁ of 4.0 g/lOmin or more (measured at 125°C under a load of 21.6 kg) and a conductive filler,

- providing and melt mixing in an extruder a polyolefin having a melting point at least 25°C higher than the melting point of the polyethylene of the first electrically semiconductive composition;

(b) applying on a plurality of conductors by coextrusion,

- a meltmix of the first electrically semiconductive composition obtained from step (a),

- a meltmix of the second electrically semiconductive composition obtained from step (a),

and coextruding onto the first or second semiconductive composition a meltmix of the polyolefin having a melting point at least 25°C higher than the melting point of the polyethylene of the first electrically semiconductive composition,

to form a film having a core layer comprising a plurality of conductors embedded within and in contact with a first layer formed by said first and second electrically semiconductive compositions and a second layer in contact with said first layer comprising said a polyolefin having a melting point at least 25°C higher than the melting point of the polyethylene of the electrically semiconductive composition.

16. A process as claimed in claim 15 wherein the film is produced using cast film extrusion and static electricity is used to hold the edges of the film at the desired width.

17. An article comprising a film as claimed in claim 1 to 15, e.g. heated clothing or a vehicle seat.

18. Use of a film as claimed in claim 1 to 15 to heat an object (e.g. in heated clothing or heated vehicle seats) or to heat an environment (e.g. a room) where the applied voltage (preferably DC) to said film is within the range of from 10 to 70 V, such as from 12 to 40 V.

Drawing